Several murine retrovirus DNA genomes have been molecularly cloned in an EK2 certified prokaryotic system. The genomes include three covalently closed circular DNAs of Harvey sarcoma virus (Ha-SV), a covalently closed circular DNA of Friend murine leukemia virus (F-MuLV), and two AKR MuLV genomes integrated in mouse cell DNA. In a mouse cell tissue culture system, the Ha-SV DNA induces foci of transformed cells, the F-MuLV DNA induces a virus which induces erythroleukemia in newborn mice with a latent period of less than one month, and the integrated AKR MuLV DNA induces infectious virus. These molecules are being used as probes to study the origin, structure, and function of specific regions of the viral genomes and their flanking sequences.